1. Field of the Invention
The present invention relates to a method and apparatus for effecting separation of airborne particulate matter by size and/or density through the use of centrifugal force.
2. The Prior Art
Many industrial situations require the separation of airborne particulate matter on the basis of size or density. For example, in the mining industry, dry milling of rock requires closed circuit recirculation of over-sized particles for regrinding. It is, therefore, necessary to separate the over-sized particles from the smaller ground particles which are allowed to pass from the grinding phase of processing.
The separation of airborne particulate matter is also an important and highly desirable component of steam boiler systems which are fired by pulverized coal. Large scale coal fired steam boiler systems are very commonly used in the generation of electrical power by electric utilities. As will be discussed hereafter, the completeness of the separation of undesirable and inert materials from the pulverized coal can have a direct effect upon the operating efficiency and the economics of the electric generating plant.
Coal is not a uniform substance but is a mixture of components that vary in both physical and chemical composition. There are two major types of impurities in coal which are of particular concern to industries which use coal as a fuel: (1) impurities which constitute what is known as ash, and (2) impurities which contain sulfur. If these impurities could be removed from the coal prior to its combustion great savings could be realized both economically and environmentally.
By removing the ash impurities, the purity of the coal is greatly enhanced so that there is more complete burning of the fuel. Furthermore, greater efficiency in the operation of the boiler system is obtained because it is not necessary to heat the ash impurities up to the combustion temperature of coal. In addition, by removing sulfur-containing impurities, the effluent gases resulting from the combustion of the coal fuel are much cleaner, thus requiring less expensive and sophisticated means for recovering and removing impurities from these effluent gases.
In the past, numerous attempts have been made to provide a system for separating particulate matter for applications such as those described above. Many of these methods have used centrifugal force as part of the means for separating the impurities. See, for example, U.S. Pat. No. 496,897 to Rathbun and Australian Pat. No. 19,197/34 to Lawrie et al.
The use of centrifugal force in the separation process is particularly well suited for applications requiring the use of coal. This is because it is well known that the impurities associated with the coal are generally more dense than the coal itself. However, the devices and methods of the prior art have been only partially successful, at best, in separating the larger and more dense particles (that is, having a greater mass) from the smaller and less dense particles (that is, having a lower mass), as is required in the above-described processing of coal. The prior art devices and methods have either left some of the larger or more dense impurity particles in the system, or have collected an unacceptably large portion of the smaller, less dense coal particles along with the larger or more dense impurity particles.
The problems experienced by devices which have used centrifugal force in the past have been varied, but the inadequate results have mainly stemmed from two important factors. First, when acted on by centrifugal forces, all particles having density or appreciable size are caused to migrate toward the outer portion of the curved surface along which they are traveling. Although the larger and more dense particles tend to migrate to this surface faster than the lighter and less dense particles, there is no clear separation point between the layers of particles which are desired to be separated from the layers of particles which are desired to be retained. This problem is magnified by the fact that as total concentrations of particles change (for example, the percentage of impurities in the coal vary), the depth of particle layers along the outer surface also changes.
The other major factor which has hampered prior art devices and methods in separating particles is that, as particles are accelerated through an enclosed, curved conduit, pressures within the conduit are increased. As a result, a significant pressure differential is created between the interior of the conduit and the exterior atmospheric pressure. Many prior art devices for separating particles by use of centrifugal force have generally involved the extension of a blade member through a port in the outer conduit surface into the pathway of particles traveling within the conduit. The blade member deflects particles near this outer surface through an exit port which opens into a collection chamber at atmospheric pressure. Unfortunately, the effects of the pressure differential between the conduit interior and the atmosphere are seen through the port.
The result of this pressure differential is that, in addition to transmitting the larger sized impurity particles through the port, air also flows from the higher pressure conduit interior to the lower pressure collection chamber. The air rushing through the port naturally draws with it many smaller and less dense particles which are not intended to be included in the separated particlate matter. This undesirable removal of the lighter particles greatly reduces the ability of these systems to improve the operating economy and efficiency associated with large, coal burning furnaces.
Furthermore, in situations of higher impurity particle concentration, it is often the case that significant quantities of impurities remain in the system because these impurities may pass beyond the reach of the inserted blade member even though the outward flowing air still carries away the lighter and less dense particulate matter. These and other problems experienced in the prior art can be more clearly understood by reference to specific prior art devices which are described in greater detail hereinafter.
One attempted method for solving the problem caused by the varying concentrations of particulate matter has been to employ blade means whose length of extension into the conduit interior is adjustable. See, for example, U.S. Pat. No. 2,044,915 to Mosley. Although only manual adjustability has been provided in the past, even automatic adjustability would not provide a full resolution of the problem. This is because there is no identifiable separation level between the larger or more dense particles and the smaller or less dense particles. In addition, such prior methods have also failed to even address the pressure differential problem.
Recognizing, at least to a limited extent, the problems caused by the pressure differential at the extraction port, several separating devices have attempted to solve the serious problem, albeit unsuccessfully, of the loss of the smaller or lighter particles due to the air blast through the port. Such devices have utilized the transmission of a stream of air into the interior of the conduit near the extraction port so that the smaller, less dense particles are deflected away from the port opening by the incoming stream of air. Examples of such proposed devices are seen in U.S. Pat. Nos. 1,505,741 to Stebbins and 1,023,750 to Morscher. The problem with this attempted solution is that it not only deflects the smaller, lightweight particles from the opening, but it also tends to deflect some of the larger particles, thereby reducing the efficiency of the separation process. In addition, there are significant costs in the operation of a device which uses such an incoming stream of air.
As is apparent from the above discussion, what is needed in the art is a device and method utilizing centrifugal force for the separation of larger or more dense particles from smaller or less dense particles in a conduit, without being subject to the adverse effects of a pressure differential at the extraction point. A further improvement in the art would be to provide a system which is capable of separating substantially all of the more dense impurity particles without additional loss of the smaller coal particles. Such a device and method is disclosed and claimed herein.